What do i say to her?

Ok i met this girl named Danielle. She works with me. When we met she told me right way that her and her BF were boring and she was kinda looking for a new guy. Well in time she told me she liked me but loved her BF. We went on dates is what i called them. Soon we were kissing and making out, ....boyfriend girlfriend stuff (i no it sounds like we were moving fast and in a way we did. But, we didn't do any of that stuff till after hours and hours each night for two weeks, of nonstop talking) Well after a month or two of her saying she wanted me but at the same time her BF she told me she loved me, and she just couldn't pick the guy she wanted. She picked her BF and now I'm stuck in love with the girl. She told me that she was infatuated with me and not in love. Also on the side note here, this BF is first girlfriend is Danielle and they ben dating for almost two years. Just thought you should no that. Well heres were i need help.You no when you brake up and move on, the other partner still wants you. Like when they see that you haved moved on they miss being the one that made you happy. Well she said she has no feelings for me in anyway. So I'm going to move on but if she comes back to me and says she was wrong and dose have feelings. What do tell her. I want her back if she ever did come back to me. So what do i say when she says i was wrong with how i feel tord you, i realy want you back. I want to say something so she will want me and come to me. In other words so she will be my girl. Were both in are 20s so this is not a little kid relationship.